Transparent glass-ceramic materials with a low thermal expansion coefficient (CTE), which contain a solid solution of β-quartz as the principal crystalline phase, have been described in a number of publications, in particular by W. Hoeland and G. Beall, in “Glass-ceramic technology”, Am. Ceram. Soc., Westerville (2002), pages 88-96. Said glass-ceramic materials are generally obtained by heat treating a precursor glass (more conventionally a mixture of the constituents of said glass: a mineral charge, a precursor of such glass), the composition of which is of the LiO2—Al2O3—SiO2 (LAS) type. Said heat treatment includes a nucleation stage followed by a crystal growth stage.
The manufacture of articles made of β-quartz glass-ceramic conventionally includes the three principal successive steps: a first step of melting a batch material, such as a mixture of raw starting materials and/or glass cullet, usually carried out between 1550° C. and 1750° C.; a second step of cooling and forming the molten glass obtained into desired shape; and a third step of crystallizing or ceramming the shaped cooled glass by a suitable heat treatment (including the phases of nucleation and crystal growth mentioned above).
Glass-ceramic materials comprising β-quartz solid solution as the predominant crystalline phase, having various degree of transparency and being essentially colorless, were known. For example, U.S. Pat. Nos. 3,252,811, 3,977,886, 4,093,468, 5,017,519, and 6,750,167, as well as DE 1,496,497, JP 06-96460 and JP 2001-348250 are all concerned with such glass-ceramic materials. Particularly, the presently claimed invention may be considered as an improvement to the one described in JP 06-96460. The ranges given in said document are large, i.e. the one for SiO2 and there is no specific teaching with regard the content of MgO+ZnO. All the exemplified compositions include As2O3, some of them include P2O5. In contrast to the teaching of JP 06-96460, the inventors of the present invention found a compositional range where, explicitly, no As2O3 is needed for fining and besides other measures, particularly a specified amount of MgO and ZnO is used to facilitate the melting process and improve optical performance.
Current commercial products of essentially transparent, essentially colorless glass-ceramic tend to have a undesirable color tint in the visible spectrum. There remains a need of a transparent, colorless glass-ceramic comprising β-quartz solid solution as the predominant crystalline phase.